Docs @ Psychtoolbox Wiki : SetDestinationAlpha

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[newSourceMat, newDestinationMat]=SetDestinationAlpha(destinationFactorStr, alpha, sourceImage, destinationImage)

Given an alpha blending factor, "destinationFactorStr", insert "alpha"
into the source or destination image such that on alpha blending with the
specified destinationFactorStr "alpha" is selected as the destination

For some combinations of destination factor string and source factor
string, SetDestinationAlpha may undo a subsequent call to SetSourceAlpha
or be undone by a previous call to SetSourceAlpha. This happens when
both the destination factor string and the source factor string specify
the same location of alpha values yet store different alpha values. For
example, if both the source alpha and destination alpha are drawn from
the source alpha plane (GL_SRC_ALPHA) then setting the source alpha to 0
would also change the destination alpha to 0. Both source and destination
would share the source alpha plane for storage of the alpha value.

SetAlphaDestination helps test OpenGL alpha blending precision.

see also: PsychAlphaBlending

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